An Ohio judge declared another mistrial in the case of a white policeman who shot and killed an unarmed black man during a routine traffic stop in July 2015.

It is the second time a mistrial has been declared in the case of former University of Cincinnati campus police officer Ray Tensing who is accused of killing Sam DuBose in an incident tinged with the same overtones of racial discrimination and heavy-handed tactics as other recent high-profile US police shooting cases.

The jurors had said on Friday that they were unable to reach a verdict in Tensing's trial, but the judge had sent them back to try again on the counts of murder and voluntary manslaughter.

Instead, they sent her another note some three hours later, saying: "We are almost evenly split regarding our votes." The note said they didn't foresee reaching a unanimous verdict.

The first trial in 2016 ended in a mistrial after the jury failed to reach a unanimous verdict.

The NAACP of Cincinnati blasted the hung jury result and said they will demand justice.

"The message that is being sent is, if you are black, all the police officer has to do is say they were in fear of their life and they get away with murder because the victim (is) black," the local NAACP said in a statement.

This jury had nine white people and three black people. His first trial had 10 white people and two black people.

Prosecutors will have to decide whether to try Tensing for a third time. A spokeswoman for the county prosecutor, Joe Deters, said he won't comment until next week.

High-profile shooting cases

The prosecutor during the first trial, Joseph Deters, had strongly criticised the shooting.

"This is the most asinine act I have ever seen a police officer make," he said at the time. "It was totally unwarranted."

US prosecutors have found it difficult to convict police officers in high-profile shooting cases involving black victims - despite recent incidents being captured on video.

Anger over the failure to find the officers guilty has sometimes boiled over, leading to violent protests.

Peterson Mingo, a DuBose family spokesman and Cincinnati church pastor, appealed on Friday for calm.

"Right now, the family wants peace. The family wants reconciliation," he told reporters.

"The family does not want any upheaval. They don't want any violence," Mingo said.

Minnesota police officer Jeronimo Yanez was found not guilty last week in the shooting death of motorist Philando Castile, whose dying moments were live-streamed on Facebook.

And all six Baltimore officers charged over the 2015 death of Freddie Gray, due to spinal cord injuries suffered in the back of a police van, were eventually cleared.

The University of Cincinnati fired Tensing in 2015 after his indictment. It restructured its public safety department and made other policing reforms. The university reached a $5.3m settlement with DuBose's family, including free undergraduate tuition for DuBose's 13 children.

To convict Tensing of murder, jurors had to find he purposely killed DuBose. The charge carries a possible sentence of 15 years to life in prison.

The voluntary manslaughter charge means killing during sudden passion or a fit of rage. That carries a possible sentence of three to 11 years.

More than 1,000 killed in 2016

According to the Guardian newspaper's The Counted database, at least 1,092 people were killed by police in the United States last year.

Nearly a quarter of those killed were African Americans although the group accounts for roughly 12 percent of the total US population.

Civil rights campaigners and activists say the disproportionate number of black Americans killed by police is part of a broader pattern of racial discrimination in the country's justice system.

According to a watchdog group The Sentencing Project, African American men are six times more likely to be incarcerated than white men.

People of colour make up around 67 percent of the 2.2 million people in US prisons and jails.

These disparities, particularly the killing of African Americans by police, has prompted the rise of the Black Lives Matter movement, a popular civil rights movement aimed at ending police violence and dismantling structural racism.

On Friday, a grand jury in Oregon declined to indict a police officer who killed a 24-year-old man, who was allegedly armed with a knife.

Court documents from recent arrests said Terrell Kyreem Johnson, who died of multiple gunshot wounds, was African American.

The jury found that the officer involved used a "lawful exercise of self-defense".

On Tuesday, hundreds of activists and mourners gathered in Seattle to protest the police killing of Charleena Lyles, a 30-year-old African American woman, who was pregnant at the time of her death.

Police claimed that Lyles, who had a history of mental illness, had been armed with a knife. But her relatives and critics questioned why the officers did not use non-lethal weapons.